1. Field of the Invention
The present invention relates generally to scanning electron microscopes for observation of cross sections and methods of observing cross sections employing the scanning electron microscopes and, more particularly, to a scanning electron microscope to be employed for observing a cross section of semiconductor wafer by directing an electron beam to the cross section thereof and to a cross section observing method employing such a scanning electron microscope.
2. Description of the Background Art
For failure analysis and process evaluation of semiconductor integrated circuits, a scanning electron microscope (hereinafter referred to as SEM) has conventionally been known as a device to be employed for observation of a surface and a cross section. In a cross section observing method employing the SEM (Scanning Electron Microscope), the method has been adopted in which wafer is cut so that a desired cross section of wafer can be observed.
FIG. 7 is a perspective view of semiconductor wafer which is required when a cross section observing method employing a conventional SEM is adopted. The cross section observing method employing the conventional SEM 5 will be described with reference to FIG. 7. A plurality of chips are formed on semiconductor wafer 27. First, a test chip 28 for cross section observation is selected to carry out cross section observation. Semiconductor wafer 27 is then cut so that the cross section 129 of test chip 28 to be observed may appear. In this manner, the cross section 129 of test chip 28 to be observed is observed by employing the SEM after exposed. As described above, conventionally, semiconductor wafer 27 has been cut manually so as to expose cross section 129 of test chip 28.
As aforementioned, in the method of observing the cross section of the semiconductor wafer employing the conventional SEM, semiconductor wafer 27 is manually cut so as to expose cross section 129 of test chip 28 in cross section observation. In the method of cutting semiconductor wafer 27, it is extremely difficult to accurately expose a specific portion of the cross section since potions corresponding to cross section 129 to be observed are cut manually. This method has another disadvantage of requiring a long time for fabrication of samples and for observation of the cross section. Since the semiconductor wafer is cut in order to carry out the cross section observation, the cut wafer cannot be subjected to a subsequent wafer process. 5 Thus, a further disadvantage has been provided that wafer for cross section observation must be prepared for each wafer process. That is, in the cross section observing method employing the conventional SEM, since the wafer is cut manually, a specific portion of the cross section cannot be accurately exposed, and the observation of the cross section requires a long time. Moreover, a wafer for observation of the cross section is required for each wafer process.
Thus, as an improvement of the cross section observing method employing the convention SEM, such a cross section observing method has conventionally been proposed that employs a scanning electron microscope for cross section observation which includes an FIB (Focused-Ion-Beam) column combined with an SEM column. This is disclosed in J. Appl. Phys. 62(6), Sept. 15, 1987, pp. 2163-2168. FIG. 8 is a schematic diagram showing the configuration of the proposed scanning electron microscope for cross section observation. Referring to FIG. 8, the scanning electron microscope includes an SEM column 100 for directing an electron beam 30 to a sample 27 to scan the sample, an FIB column 200 for generating an ion beam 31 to scan sample 27, a photomultiplier 23 for detecting secondary electrons emitted from sample 27 by scanning with electron beam 30 and ion beam 31, and an observation CRT 26 connected to photomultiplier 23. In a cross section observing method employing this scanning electron microscope, electron beam 30 and ion beam 31 are first made in registration with each other at a reference mark. Then, a place to be processed is observed with electron beam 30 and then processed with ion beam 31. The employment of this scanning electron microscope for cross section observation has an advantage that it is unnecessary to manually cut sample 27 (semiconductor wafer) for observation of the cross section.
However, the proposed scanning electron microscope fails to perform observation by SEM simultaneously with a process by FIB because the SEM-observation is carried out by employing the secondary electrons generated from the semiconductor wafer (sample 27). More specifically, since the secondary electrons are generated from the semiconductor wafer (sample 27) in both FIB-scanning and SEM-scanning of the semiconductor wafer, it is impossible to detect only the secondary electrons by the SEM-scanning even though the cross section observation is carried out by SEM at the same time when cutting the cross section of the semiconductor wafer (sample 27) with FIB. FIGS. 9A-9E are schematic diagrams for explaining a process for cutting a cross section to be observed by the proposed scanning electron microscope. Referring to FIGS. 9A-9E, in the proposed scanning electron microscope, it is necessary, when cutting the cross section to be observed, to repeat the step of cutting a predetermined amount of the cross section with FIB as shown in FIG. 9A, and then observing the cross section by SEM as shown in FIG. 9B (FIGS. 9A-9E). Accordingly, there is a disadvantage that it is difficult to reduce the time required to cut the cross section to be observed. That is, the intervals between the processes by FIB shown in FIGS. 9A and 9C and the observations by SEM shown in FIGS. 9B and 9D require approximately several seconds in general, so that a continuous observation cannot be carried out. As one countermeasures, such an approach is considered that reduces the intervals by multiplexing a control system of FIB and SEM; however, this approach results in a disadvantage of complexity of the apparatus. In addition, since the cutting of the cross section to be observed with FIB cannot be carried out simultaneously with the observation by SEM, it is difficult to obtain more accurately a specific portion of the cross section to be observed.